Preparation of a copy hologram with a light modulating array derived from a master hologram

ABSTRACT

A method of preparing a copy hologram from a master hologram in which the light employed in the copying process is modified by passing an exposing beam through a light modulating array. At least some members of the array have modified light transmissivities in accordance with information derived from a reconstruction of the master hologram.

This invention relates to the copying of holograms. It is usual toproduce copies of reflection holograms from a reflection master hologrambut recently a method has been found whereby reflection copies ofholograms can be prepared from a transmission master hologram. Suchmethod is described in our copending U.S. Pat. application, Ser. No.07/308,662, entitled "Contact Copying of Reflection or VolumeHolograms." However, whatever the method employed, very often the masterhologram when reconstructed exhibits undesired highlights or areas whichare too dark. It is not possible to cure such faults during the copyingprocess by the burning-in or dodging techniques employed when preparingpositive prints from negatives in the usual photographic enlargingmethod.

We have found a method of reducing the undesirable effects of highlightsand dark areas present in master holograms when copying hologramstherefrom.

Therefore, according to the present invention there is provided a methodof preparing a copy hologram from a master hologram which ischaracterized in that the light employed in the copying process ismodified by passing at least one exposing beam through a lightmodulating array, some members at least of said array having had theirlight transmissivities modified in accordance with information derivedfrom a reconstruction of the master hologram.

Preferably, the light modulating array is an array of liquid crystalpixels known as a liquid crystal display device (LCD). However, usefullight modulating arrays may be made from other materials such as thosedescribed in U.S. Pat Nos. 4,375,649 and 4,378,568, these entiredisclosures being incorporated herein by reference. Some of thematerials described in these specifications are electro-opticalmaterials transformable by an electric field from a non-polar, isotropicstate to a polar birefringent state.

Preferably, the optical density of each member or pixel in the lightmodulating array can be altered to give a range of light transmissiondensities.

The light modulating array is preferably an X-Y array when an expandedbeam is used as the exposing beam. The light modulating array ispreferably an X array when a scanning beam is used as the exposing beam.

The method of the present invention may be used to produce reflectionhologram copies from a transmission master or to produce reflectionhologram copies from a reflection master.

In a particularly useful method of producing reflection copies from atransmission master, a laser beam is split to form an object beam whichpasses through the transmission master and is modified thereby andenters the light-sensitive holographic material where it interferes withthe reference beam to form the holographic interference fringes. In thismethod, preferably, it is the object beam which is modified by passingit through the light modulating array. However, the reference beam couldalso be so modified or indeed both the object and reference beams couldbe so modified.

When a reflection hologram is copied from a reflection master, usuallythe light-sensitive holographic material is placed substantially incontact with the reflection master and the laser beam passes through theholographic material into the reflection master where it is modified bythe holographic image therein and is reflected back into the holographicmaterial so forming the object beam.

Thus, in this case, preferably, the laser beam which is used to exposethe holographic material passes through the light modulating arraybefore it enters the holographic material. However, if a very thin lightmodulating array is used, then this can be sandwiched between theholographic material and the reflection master so that the object beammay be modified by the light modulating array.

One useful method of modifying the light transmissivities of the lightmodulating array members in accordance with information derived from areconstruction of the master hologram is to reconstruct the masterhologram and then prepare video signals representative of this image byuse of a video camera placed along the axis to be used for viewing theholographic copies, digitizing the video signals and storing them andthen using the stored signals to modify the elements of the lightmodulating array.

The stored signals may be fed to a visual display unit such as atelevision screen via a transforming unit, and a reconstruction of themaster hologram in two dimensions is then displayed. This will show upall the undesirable highlight and dark shadow areas in the reconstructedimage of the master hologram. This image can then be corrected by use ofa keyboard of a mouse to alter the transformation applied to the imagebefore display. This could be either on a point-by-point basis or byaltering the numerical algorithm which relates the appearance of thereconstructed hologram to the appearance of the displayed image.

This alters in real time the image on the television screen. When theviewed image on the television screen appears to be satisfactory, thestored signals from the transforming unit are fed to the lightmodulating array and there they modify its light transmissivityselectively. The master hologram is then used to print copies by usinglaser light to pass through the master, at least one exposing beam beingmodulated by passing it through the light modulating array which has hadits transmissivity selectively modified, thus compressing the highlightand dark shadow tonal values to achieve a better match between themaster hologram and the photosensitive copy material. Preferably thedigitized video signals are stored on a disc, but they may be stored byany convenient digital or analog means such as a matrix array.

In this method of modifying the array of light values as just described,an operator looks at the reconstructed two-dimensional representation ofthe holographic image and decides how it should be modified. However,when the sensitometric characteristics of the photosensitive material tobe used as the copying material are well known, it is possible to employa socalled look-up table which has been calibrated using thesecharacteristics. In this case, the stored signals are passed to thetransforming unit in the apparatus and the results read off by the"look-up table" which automatically modifies the transmittances of someof the elements of the light value array without an operator playing anypart in the process.

The method of the present invention is preferably used when reflectionhologram are copies from a transmission master. In one of our co-pendingapplications, there is described a method of preparing a reflectionhologram which comprises mounting a master transmission hologram inregister with a sheet or length of transparent photosensitiveholographic material, there being present therebetween means to cut outzero order light transmitted through the transmission hologram, therefurther being present on each side of the assembly which comprises thehologram and the holographic material a light reflecting surface, thencausing a split beam from a laser source to expose both the wholesurface of the transmission hologram and the holographic material bydirecting the beams on to both reflecting surfaces and on passingthrough the transmission hologram, said one beam takes up the imageinformation and passes through the means to cut out the zero order lightto the transparent photosensitive holographic material where said onebeam interferes with the other beam which is reflected from the otherreflecting surface in to the holographic material, the two splitportions of the beam being maintained in registration, and thenprocessing the holographic material to fix the holographic imagetherein.

In one method, the split beam from the laser source scans over both thewhole surface of the transmission hologram and the holographic material.

In another method, the split beam is derived from a laser source whichprovides a single overall exposure of the photosensitive holographicmaterial. Preferably this laser source is a pulsed laser source as thisgives the shortest exposure time. A suitable laser for use in thismethod is a pulsed ruby laser.

This latter method is shown in the accompanying FIG. 1, whichillustrates the method of the present invention.

FIG. 2 shows the flow diagram of the pieces of equipment required tomodify the light modulating array which in this case is a liquid crystalmatrix.

According to another aspect of the present invention, there is providedan apparatus for carrying out the method as just described whichcomprises a means for reconstructing a master hologram, means forpreparing video signals representative of the reconstructed hologram,means for digitizing these video signals and storing them, a lightmodulating array and means for altering the transmissiveness of membersof the light modulating array based on the information derived from thestored signals.

Preferably the means for preparing video signals representative of thereconstructed hologram is a video camera.

Preferably viewing means are provided for displaying a reconstruction ofthe master hologram in two dimensions. Most preferably this viewingmeans is a television screen.

Preferably the digitized video signals are stored in a disc. Suitablediscs include magnetically or electrically encoded computer (floppy)discs, or optically encoded compact discs of the type that may be readwith a laser light beam.

The accompanying figures will serve to illustrate the present invention.

FIG. 1 is a front side view of a holographic copying apparatus.

FIG. 2 is a block diagram of the apparatus used to modify the members ofthe light modulating array with regard to their transmissiveness.

In FIG. 1, a pulsed ruby laser source 21 produces a laser beam 22 whichis directed through a beam splitting device 25 which splits the beam 22into two beams 26 and 27. Beam 26 is directed towards a silvered concavemirror 29 via a beam expanding means 34 and is reflected from the mirror29 via a liquid crystal matrix 28 through a transmission hologram 30 andthen via a louvre filter 31 (which cuts out zero order light) into oneportion 32 of a long length of silver halide sensitized holographic filmmaterial 33. Beam 27 is directed towards a silvered concave mirror 29via a beam expanding means 34 and is reflected from the mirror 29 intothe same portion 32 of the film material 33 where it interferes withbeam 26 during the overall single pulse emitted by the laser source 21.

Beam 26 can be considered to be the replay beam or object beam as itcarries the information from the transmission hologram 30 into the filmmaterial 33 and beam 27 can be considered to be the reference beam.Beams 26 and 27 are in registry when they interfere in the film material33.

The unexposed film 33 is stored on the spool 35 and the exposed firm isreeled up on the spool 36. Spool 36 is a driven spool and it rotates toreel up a predetermined length of exposed film 33 after each pulse fromthe laser source 21 and at the same time causing an equivalent length offilm 33 to be unreeled from spool 35. This ensures that at each pulseexposure a fresh portion 32 of unexposed film 33 is present below thelouvre filter 31.

After all film 33 has been exposed and reeled up on spool 36 the exposedfilm can be processed to fix the holographic image fringes therein.

The set-up as described with reference to FIG. 1 is of particular use inproducing a plurality of copies of the hologram 30 on the length ofphotosensitive film 33 but of course the hologram 30 can be changed atany time during the exposure sequences.

With reference to FIG. 1, it is to be understood that the optics aresuch that the beam 27 coming from the beam splitting means 25 does notcross directly the path of the photosensitive material 33.

In FIG. 2, there is shown the hardware and sequence of events used tomodify the liquid crystal matrix 28.

In FIG. 2, there is shown the transmission hologram 30, a video camera40, a charge coupled device 41 (CCD) which is present in the camera 40,an analog/digital convertor 42, a disc storage means 43, a numericalalgorithm 44, a television screen 45, a keyboard 46, a display means 47and the liquid crystal matrix 28.

In operation, before the transmission hologram 30 is exposed on to thefilm material 33 as just described, the transmission hologram isreconstructed using laser light. The video camera 40 is placed along theaxis at which the copy hologram should be viewed, then it captures atwo-dimensional image of the reconstructed image in the hologram 30.This is recorded in the CCD 41, and the recorded signals are passed viathe A/D converter 42 to the disc storage means 43. The stored signals ondisc 43 can then be fed to the television screen 45, on which can beseen the two-dimensional image of the reconstructed hologram in thetransmission hologram 30 via the numerical algorithm 44.

The two-dimensional image of the reconstructed hologram will show up anyundesirable highlights or dark shadow areas. The keyboard 46 is thenoperated to remove such highlight or dark shadows via the numericalalgorithm 44.

Thus, when the laser beam 26 exposes the photosensitive film material 33via the concave mirror 29 and the transmission hologram 30, it will havebeen modified by the liquid crystal matrix 28 so that when the material33 has been exposed and processed the reconstructed hologram willexhibit no undesirable highlight nor dark shadow areas because ofcompression of the tonal values. It is to be understood that thefunction of the numerical algorithm 44 is to transform stored imageinformation to information having a smaller dynamic range.

The numerical algorithm may be connected to a "look-up table" which iscalibrated with the sensitometric properties of the film material 33 andthus may function to reduce the dynamic range of the stored imageinformation in conjunction with an operator or automatically without theassistance of an operator.

It is to be understood that the keyboard 46 may be replaced by aso-called mouse to alter the image of the television screen 45.

We claim:
 1. A method of preparing a copy hologram from a master hologram, in which the light employed in the copying process is modified by passing at least one exposing beam through a light modulating array comprising an array of liquid crystal pixels, at least some members of said array having had their light transmissivities modified in accordance with information derived from a reconstruction of the master hologram.
 2. A method according to claim 1 characterized in that the optical density of each pixel in the array can be altered to give a range of light transmission densities.
 3. A method of preparing a copy hologram from a master hologram, in which the light employed in the copying process is modified by passing at least one exposing beam through a light modulating array, at least some members of said array having had their light transmissivities modified in accordance with information derived from a reconstruction of the master hologram, wherein video signals representative of the reconstruction are obtained by directing a video camera along the axis to be used for viewing the holographic copies, digitizing the video signals and storing the digitized signals, to provide the information for modifying the elements of the light modulating array whose light tramsmissivities are to be modified.
 4. A method according to claim 3 characterized in that the stored signals are fed to a visual display unit via a transforming unit and a reconstruction of the master hologram in two dimensions is then displayed.
 5. Apparatus for preparing a copy hologram from a master hologram comprising:means for reconstructing a master hologram, means for preparing video signals representative of the reconstructed hologram, means for digitizing the video signals and for storing the digitized video signals, a light modulating array, means for passing at least one exposing beam through the light modulating array to prepare the copy hologram, and means for altering the transmissiveness of members of the light modulating array based on information derived from the stored digitized video signals.
 6. An apparatus according to claim 5 characterized in that the means for preparing video signals representative of the reconstructed hologram is a video camera.
 7. An apparatus according to claim 5 characterized in that there is provided viewing means for displaying a reconstruction of the master hologram in two dimensions.
 8. An apparatus according to claim 5 characterized in that the light modulating array is a pixel array of liquid crystals.
 9. A method of preparing a copy hologram from a master hologram, in which the light employed in the copying process is modified by passing at least one expanded exposing beam through an X-Y light modulating array, at least some members of said array having had their light transmissivities modified in accordance with information derived from a reconstruction of the master hologram. 